1. Field of the Invention
This invention relates to a power supply circuit converting an alternating current voltage into a direct current voltage.
2. Description of the Related Art
A power supply circuit having a rectifier circuit (a diode bridge circuit, for example) and a smoothing circuit (a capacitor, for example) and converting an alternating current voltage into a direct current voltage is known as a power supply circuit for various kinds of electronic equipment. However, such a power supply circuit has a poor power factor when an alternating current voltage of a sine wave is applied, because an electric current flows through the power supply circuit only around a peak of the sine wave. Also, in the case where a current not proportional to the sine wave flows although the alternating current voltage of the sine wave is applied, higher harmonics are caused to exert an adverse effect on peripheral equipment in some cases.
Thus, there is proposed a power supply circuit which improves the power factor, suppresses the higher harmonics, and regulates the direct current voltage by connecting a switching device in parallel to the diode and switching the switching device at an appropriate timing to make the current flowing through the power supply circuit similar in shape to the sine wave of the alternating current voltage (Refer to Japanese Patent Application Publication No. 2006-25579, for example.).
FIG. 10 is a block diagram showing a structure of a conventional power supply circuit. The power supply circuit is provided with four diodes D1, D2, D3 and D4, which constitute a diode bridge circuit to full-wave rectify an alternating current voltage, a resistor RX for current detection, switching devices 116 and 117, a drive circuit 124, a switching control circuit 126 and divider resistors R1 and R2 to divide an output voltage.
The alternating current voltage from an alternating current power supply 110 is applied through a reactor 112 to an R line that is an alternating current input line, while the alternating current voltage from the alternating current power supply 110 is directly inputted to an S line that is another alternating current input line. The alternating current voltage applied by the alternating current power supply 110 is full-wave rectified by the diodes D1, D2, D3 and D4, before being smoothed by a capacitor 123 connected between a P-side electrode and an N-side electrode.
The switching device 116 is connected in series with the resistor RX. The serially-connected switching device 116 and the resistor RX are connected in parallel with the diode D1. The switching device 116 and the diode D1 are connected in reverse polarity (directions of the current flow are opposite to each other).
The switching device 117 is connected in series with the resistor RX. The serially-connected switching device 117 and the resistor RX are connected in parallel with the diode D2. The switching device 117 and the diode D2 are connected in reverse polarity (directions of the current flow are opposite to each other). The resistor RX detects a current flowing through either the diode D1 or the diode D2 in a period during which either the switching device 116 or the switching device 117 is turned on.
The divider resistors R1 and R2 are connected in series between the P-side electrode and the N-side electrode, and divide the output voltage generated between the P-side electrode and the N-side electrode. That is, a divided voltage signal of the output voltage is outputted from a connecting node between the divider resistors R1 and R2.
The drive circuit 124 amplifies an input from the switching control circuit 126 to large enough amplitude to drive the switching devices 116 and 117 before outputting it. The switching control circuit 126 controls timings to turn on and off the switching devices 116 and 117 based on results of the current detection by the resistor RX and the divided voltage signal of the output voltage.
Next, operations of the conventional power supply circuit are explained. FIG. 11 shows the operations of the power supply circuit in the case where the alternating current voltage of the alternating current power supply 110 on the R line is positive relative to that on the S line.
<In the Case where the Switching Device 116 is Turned On>
In the case where the alternating current voltage on the R line is positive relative to that on the S line and the switching device 116 is turned on, a current flows through a path indicated by a chain line in FIG. 11, that is, the R line (the reactor 112)→the switching device 116→the resistor RX→the diode D2→the S line. Energy is stored in the reactor 112 during this period.
<In the Case where the Switching Device 116 is Turned Off>
In the case where the alternating current voltage on the R line is positive relative to that on the S line and the switching device 116 is turned off, the reactor 112 works to make a current flow in the same direction as in the case where the switching device 116 is turned on. Therefore, the current flows through a path indicated by a dashed line in FIG. 11, that is, the R line (the reactor 112)→the diode D3→the capacitor 123→the diode D2→the S line, and the capacitor 123 is charged. The energy stored in the reactor 112 is outputted to the capacitor 123 to boost a direct current voltage generated between the P-side electrode and the N-side electrode (hereafter referred to as a direct current output voltage) during this period.
FIG. 12 shows the operations of the power supply circuit in the case where the alternating current voltage of the alternating current power supply 110 on the R line is negative relative to that on the S line.
<In the Case where the Switching Device 117 is Turned On>
In the case where the alternating current voltage on the R line is negative relative to that on the S line and the switching device 117 is turned on, a current flows through a path indicated by a chain line in FIG. 12, that is, the S line→the switching device 117→the resistor RX→the diode D1→the R line (the reactor 112). Energy is stored in the reactor 112 during this period.
<In the Case where the Switching Device 117 is Turned Off>
In the case where the alternating current voltage on the R line is negative relative to that on the S line and the switching device 117 is turned off, the reactor 112 works to make a current flow in the same direction as in the case where the switching device 117 is turned on. Therefore, the current flows through a path indicated by a dashed line in FIG. 12, that is, the S line→the diode D4→the capacitor 123→the diode D1→the R line (the reactor 112), and the capacitor 123 is charged. The energy stored in the reactor 112 is outputted to the capacitor 123 to boost the direct current output voltage during this period.
The power supply circuit is structured so that the current flows through the resistor RX only when either the switching device 116 or the switching device 117 is turned on. The current flowing through the power supply circuit can be made similar in shape to the sine wave of the alternating current voltage to improve the power factor by controlling the timing to switch the switching device 116 or the switching device 117 based on the current flowing through the resistor RX.
However, there is a problem with the conventional power supply circuit that a ripple current of an input/output of the power supply circuit due to the switching of the switching device 116 or the switching device 117 is large.
This invention is directed to offering a power supply circuit that is capable of improving the power factor as well as reducing the ripple current of the input/output of the power supply circuit due to the switching of the switching device.